Solid electrolyte bodies in which an electrode layer is applied thereto are frequently used as electrochemical sensors. One such application is to sense the composition of exhaust gases from internal combustion engines, typically automotive-type internal combustion engines, by determining if the exhaust gases are reducing or oxidizing. Sensors of this type, also known as lambda sensors, carry an electrode layer on at least part of their surface. It has been customary to apply the electrode layer on the presintered solid electrolyte body and then sintering the electrode layer on the body. The electrode layer, which is exposed to the exhaust gases and possibly uncovered solid electrolyte body surface, can be covered with a porous ceramic cover layer which, after its application, is again sintered. Another method of application of such a porous protective layer is by plasma-spraying technology or flame-spraying technology.
The electrodes, which are applied to a surface which is exposed usually to ambient air and which serves as a comparison electrode, customarily are not supplied with such a cover layer. Sensors which are formed as tubes which are closed at the end exposed to the gases usually carry the reference or comparison electrodes at the interior of the tube; the interior of the tube is in communication with ambient air through suitable openings in the socket or housing for the solid electrolyte body.
The electrode need not be a single layer; rather, it has been proposed already to use a system comprising a plurality of layers as the electrode, in which the layer which is closest to the solid electrolyte body itself forms the actual electrode connection. Such a layer system can have good adhesion on the body. In one method, the layer system is applied to the electrode body before sintering thereof, or before an intensive heat treatment; sinter processes in the electrode layer are then caused to occur. If a porous cover layer made of a ceramic material is already applied thereto, and this composite system is jointly sintered on the solid electrolyte body, the adhesion thereof on the solid electrolyte body, and hence of the electrode layer, will be excellent.
It has been found that sensors, and particularly those sensors which carry only a single or simple electrode layer, cannot be sufficiently loaded with electrical current. This is particularly apparent in the inner electrodes of such tubular sensors. Electrical loading capacity of the electrodes is important when the cells are cold, that is, upon starting of the engine with which the cells are associated, and hence upon starting of the sensing cycles of the sensors. The electrodes additionally can be subjected to a high electrode polarization, especially at lower temperatures, that is, at temperatures substantially below the operating temperature. Compensation for the difference in response characteristics can be obtained electronically, at the expense, however, of complex circuits.